T53E-01
Transition From Rift to Drift at Obliquely Divergent Continental Rifts: the Paired Rio Muni (W Africa) and NE Brazilian Margins
We develop a synoptic model for the breakup and drift of a major sheared continental margin system: the Rio Muni basin and its NE Brazilian counterpart. It relates the evolution of crustal structure determined from seismic interpretation (including gravity-modelling of the deep-imaging PROBE dataset) to the cooling history of this margin system yielded from thermal history data (mainly apatite fission track analysis - AFTAr - and vitrinite reflectance data). Shear margins initiate as a leaky transform fracture system accommodating the oblique (i.e. non-orthogonal) divergence of opposing rifted continental margins. As such, the transition from continental breakup (i.e. rupture) to continental drift (i.e. ocean opening) at shear margins exhibits significant differences from that of the much better understood normally divergent rifts, where the spreading vector is normal to the strike of the opposing rift margins. For example, unlike at normally divergent rifts, continental breakup and drift are separate episodes in the early evolution of shear margins. In Rio Muni-NE Brazil, they are recorded by separate breakup and drift unconformities spanning a 15-20Ma. interval, the time taken for the ocean ridge to traverse the length of the margin before a continuous arm of oceanic crust separated Rio Muni from its Brazilian counterpart. In the Rio Muni basin, the c.70km-wide Ascension Fracture Zone (AFZ) exhibits oblique-slip faulting and synrift half-graben formation that accommodated oblique extension during the period leading up to and immediately following whole-lithosphere failure and continental breakup 117Ma. Gravity-modelling of PROBE seismic profiles reveals a land-locked precursor oceanic basin that preceded full ocean opening and which subsequently was stranded on the African margin. Its existence supports the idea of multiple rift suture lines accommodating episodic breakup and it emphasizes the significance of separate breakup and drift episodes at shear margins. Oblique extension is recorded also by strike- and oblique-slip fault geometry within the AFZ and buckling of Aptian synrift rocks in response to block rotation and local transpression. Principal cooling episodes affecting both margins identified from the AFTAr occurred in the Mid-Cretaceous 110-90Ma., following a period of high heatflow during which the geothermal gradient was more than twice that of the present day 25°C/km gradient, and 45-35Ma. coinciding with a major change in the pole of rotation at Chron 34 some 84Ma. Additional cooling episodes between 85-75Ma. and 10-2Ma. are restricted to the African side. Given that this margin system underwent breakup some 117Ma., it appears that thermal equilibrium was maintained throughout the initial breakup meaning that no significant uplift and erosion is recorded by the thermal history data during this period. Instead, Mid-Cretaceous cooling is interpreted as a response to the influence of i) the c.105Ma. St Helena Plume and ii) transpressional shortening and inversion that accompanied the breakup to drift period.
T53E-02
A Multidisciplinary Investigation of Rio Grande Rift Deformation
The Rio Grande rift is the easternmost actively deforming province of the western margin of North America. Geologic observations suggest the character of rifting changes from north to south, with a narrow rift marked by linear topographic depressions in Colorado and northern New Mexico grading to a broad ''basin and range'' expression in south-central New Mexico. Reasons behind the variable character of the Rio Grande rift are not well-Understood, but may have significant bearing on our understanding of how and why continental lithosphere extends and of earthquake and volcanic hazards in rift zones. Copious geophysical data make it an ideal laboratory in which to study the roles of lithospheric rheology, plate boundary forces and local buoyancy in active continental rifting. However, existing data are ambiguous about physical processes, rates and spatial distribution of RGR extension. For example, mantle seismic expression of the rift may be narrow or wide depending on whether one assumes $V_p$, $V_s$ or $Q$ anomalies as a proxy. Moreover, estimates of strain are imprecise. Geological and geodetic data suggest rates ranging from 0.3 to 5 mm/yr extension. Existing Continuously Operating Reference System (CORS) network GPS sites suggest ~1 mm/yr extension with an additional ~1 mm/yr of left-lateral strike slip, but uncertainties are as large as the signal. In concert with the Plate Boundary Observatory--EarthScope deployment of 20 coarsely (~200 km) spaced continuous GPS in the vicinity of the Rio Grande Rift, we are preparing a focused deployment of 24 quasi-continuous, quasi-campaign GPS sites in five <50 km-spaced profiles that will serve as a platform to address several questions about the nature of continental rifting, including: What seismic hazard does the Rio Grande Rift pose? Is deformation steady or episodic? Does observed Quaternary fault slip agree with geodetic strain rates? How wide is the rift, and how does strain vary along strike? How far north has the Rio Grande Rift propagated? What is the influence of mantle rheology, and how does deformation style depend on strain rate? This presentation will review what we already know about the rift, including the range of hypothetical mechanisms for rifting, and assess what kinds of geodetic information are needed to achieve a better understanding. Recovering signal from such low deformation rates will be challenging. We will discuss progress on site selection, permitting and installation of the network as well as monumentation and occupation strategies. We will also present other advances in data analysis needed to achieve high accuracy estimates of GPS velocity in low-strain-rate locales such as the Rio Grande rift.
T53E-03
Extensional Strain Partitioning in the Eastern Black Sea From Wide-Angle Seismic Data and Subsidence Analysis
The eastern Black Sea is a deep, extensional basin that most likely opened in the late Cretaceous and early Tertiary as a back-arc basin during the subduction of the Tethys Ocean. This area provides an excellent natural laboratory for the study of extensional basins due to the thick sequence of sediments, which host a record of the subsidence history, and the proximity of the conjugate margins, which reduce uncertainties in reconstructions. Additionally, unraveling the timing and style of opening of the Black Sea, which has been the subject of considerable controversy, is critical for studying both past and present day tectonics in this active region. To this end, we employ a two-part study that includes both subsidence analysis and crustal seismology. A new onshore-offshore wide-angle seismic refraction dataset was acquired in February-March 2005 along lines coincident with industry reflection profiles. Both four-component ocean-bottom seismometers and three-component land seismometers were deployed along four transects that survey thin crust in the center of the eastern Black Sea, the Mid Black Sea High (a continental crustal ridge that separates the eastern and western basins), Sinop Trough (a basin at the edge of the eastern Black Sea), and the southern margin. Subsidence analysis has also been conducted on coincident lines by flexurally back-stripping stratigraphic horizons identified in reflection profiles and using them to invert for temporal and spatial variations in strain rate. In this presentation, we show velocity models of two 160-km-long, SW-NE oriented refraction profiles, Lines 3 and 4, obtained by first arrival tomography and coincident strain rate inversion results. Velocity models of these transects show the following new features: 1) significant, asymmetric crustal thinning beneath Sinop trough, 2) a rapid decrease in crustal thickness from ~35 km beneath the Mid Black Sea High to ~8 km in the center of the basin over a lateral distance of ~30 km, 3) a thickness of ~9 km of sediments in the center of the basin, which includes a low velocity zone at its base. The significant reduction in crustal thickness beneath Sinop trough suggests that strain was not merely concentrated in the center of this deep basin, but was partitioned over a broader area prior to localization in the basin center. Crustal thickness variations in velocity model along Line 3 imply extensional factors of ~2 beneath Sinop Trough and ~4 in the basin center. Similar degrees of extension are inferred from subsidence analysis based on stratigraphic horizons identified in the coincident reflection profiles if reasonable assumptions of paleowater depth are made. Furthermore, the shallowest occurrence of mantle velocities beneath Sinop Trough is offset laterally from the deepest part of the basin, implying an element of depth-dependent stretching. Eight-km-thick crust in the center of the basin could correspond either to oceanic crust produced in a back-arc setting or thinned continental crust; continuing work will seek to differentiate between these two possibilities. We use these observations together with subsidence analysis to present a model for the evolution of eastern Black Sea and gain generic insights into extensional basin formation.
T53E-04
Campbell Plateau, New Zealand: Seismic Analysis and Models From a Rifted Submarine Plateau of Continental Origin
Rift systems give important insights into the processes that control the beginning extension and subsequent break-Up of continents. Quantifying the amount of crustal stretching and the position of the continent-ocean boundary helps refining plate-kinematic reconstructions, as this will overcome problems in previous models which were based on rigid plate assumptions. The submarine continental plateaux off southeastern New Zealand (NZ), Chatham Rise (CR) and Campbell Plateau (CP), were adjacent to Marie Byrd Land (MBL) of Antarctica until extension and subsequent seafloor spreading formed the Southern Ocean in the Late Cretaceous. While the timing of the BT opening between CR and CP is indirectly derived from plate-tectonic evidence, the processes of extension of the Bounty Trough (BT) and the development of the fragments forming CR and CP are not yet understood. Models suggest either a rift system of the Southern Pacific's early opening or a subsequent opening of an already existing back-arc basin, a proto-BT. To investigate the evolutionary processes of these submarine plateaux, a geophysical and geological survey was conducted across CP and BT in early 2003 with the German R/V SONNE during cruise SO-169 (project CAMP). The survey carried out two deep crustal seismic transects and a series of multichannel seismic reflection lines across GSB and across BT. Velocity-depth and gravity models infer an extremely thinned crust beneath the Bounty Trough and the Great South Basin (GSB). The thickness of the crystalline crust is reduced from 20-23 km under the CR and the CP to some 12 km under the BT. Beneath the GSB, the crystalline crust thins to some 14 km from about 26 km towards CP and the South Island of NZ. P- and S-velocities are significantly increased directly beneath the Bounty Channel. We interpret a high-velocity, high-density body in the BT as a magmatic intrusion into thinned continental crust. Crustal thinning ceased shortly prior to the onset of seafloor spreading. The modelled magnitude and style of rifting along BT and GSB places constraints on reconstructing the Cretaceous break-Up process between NZ and MBL.
T53E-05
Structural Evidence from Dall Bank of Miocene Continental Rifting in the Southern California Continental Borderland
In October of 2003, intriguing northeast-trending faults displaying characteristics of rifting processes were imaged during a fish-habitat related high-resolution multibeam mapping survey of Dall Bank, a shallow submarine bank within the southern California Borderland. These structures are of extreme interest as they are aligned perpendicular to the dominant northwest-trending structural grain of the Borderland, and as such may represent an earlier phase of Borderland evolution. While the Borderland contains numerous examples of transtension-related extension including crustal thinning, unroofing of metamorphic rocks from deep levels, and possible incipient seafloor spreading, no intact rift-related structures on the magnitude of those at Dall Bank had been documented. During October of 2004 we returned to Dall Bank as part of a NOAA-Northwest Fisheries Science Center Advanced Technologies cruise, providing us the opportunity to gather additional high-resolution multibeam and backscatter imagery, 3.5 kHz sub-bottom data, ROV recovered seafloor samples, and magnetic anomaly profiles. These new data included imagery of numerous possible intact volcanic cones, samples of basaltic and andesitic volcanic rocks, and additional imagery of the extensive network of normal faults, which provide compelling structural evidence of crustal extension. These data, coupled with recently released multi-channel seismic reflection data have allowed us to extend our picture of the spatial and temporal extent of the structures at Dall Bank. We now have the ability, through the use of these new data sets, to investigate the true nature of this feature in terms of its structural construction and its significance within the broader picture of Miocene-Recent Borderland tectonics.
T53E-06
Along-axis Segmentation of the Main Ethiopian Rift from Tomographic Inversion of Local Earthquakes
Geophysical, structural, and geochemical data indicate that the East African rift system in Ethiopia is in a stage of development transitional between continental and oceanic spreading. In the northern Main Ethiopian rift (MER) the archetypal continental rift stage border fault systems bounding the asymmetric rift basins formed at ~11 Ma. After 3.5 Ma, strain localised to fault systems within the central basin. By 1.8 Ma, faulting and magmatism localised to an en echelon series of 20 km-wide zone, 50-60 km-long 'magmatic segments' comprising felsic shield volcanoes and basaltic fissural flows and dikes. As part of the international EAGLE project, 50 broadband Guralp 6TD seismometers were deployed across 3 magmatic segments, and a sector of the rift without magmatic segments. These instruments nested within the Leeds University broadband array, and the combined net of 80 instruments was used to locate local and regional events. P wave inversion for velocity with a series of synthetic tests will be presented at the meeting along with the best 1d velocity model for locating local earthquakes within the Main Ethiopian rift. Results indicate a series of high velocity zones beneath the magnetic segments at a depth of 10km.
T53E-07
Structure of the NE-Rockall Trough from Wide-Angle Seismic Data Modelling: The Role of Pre-break-Up Extension on the Formation of Continental Margins
The Rockall Trough is a 250 km wide and up to 3 km deep bathymetric basin separating the Irish and UK continental shelves from the Rockall and Hatton Banks. It is one of a series of rift basins that formed prior to the opening of the present day North Atlantic Ocean. Two wide-angle seismic lines located in the northeast Rockall Trough were acquired in May 2000. One Line crosses the Trough from the Hebrides continental shelf to oceanic crust west of Lousy Bank in a NW-SE direction. The other line intersects the first, crosses the Wyville-Thomson Ridge in a SW-NE direction and ends in the Faroe-Shetland Trough. Sonobuoy data and expanding-spread profiles acquired in the same area have been remodelled. Analysis of the seismic data using travel-times and amplitudes reveals an up 5 km thick sedimentary layer including an up to 1.5 km thick basaltic layer present in most of the trough. Beneath the sediments is highly stretched continental crust of ca. 13 km thickness. The crust thickens to about 24 km beneath Lousy Bank, which is interpreted to be of continental nature. Beneath the Hebrides continental shelf a three layer continental crust 26 km thick is modelled. A high-velocity layer up to 12 km thick is observed underneath the ocean-continent boundary west of Lousy Bank and is interpreted as magmatic underplating resulting from excess volcanism during rifting. The modelled wide-angle profiles show that the Rockall and Faroe Shetland Troughs have stretching factors of between 2 and 6. Stratigraphic data suggests that rifting in these Troughs took place in early to mid-Cretaceous times (c. 120 Ma), some 60 Ma before the opening of the NE Atlantic. Consequently, at the time of continental break-Up at about 55 Ma, the lithosphere beneath the Troughs was dominated by strong mantle lithosphere, making them resistant to further rifting. They remained strong as N Atlantic rifting gave way to ocean floor spreading and they do not show evidence of extension (e.g. faulting) of Early Paleogene age. Rifting to form oceanic crust took place along an axis to the east of the Troughs, through the relatively unextended and potentially weaker lithosphere of the Hatton Bank, Lousy Bank and Faroes Bank regions. The magmatic underplating may have played a role in the final stages of continental break-Up but its distribution suggests that it does not appear to play a significant role in modifying the crustal structure on a large scale to promote break-Up.
T53E-08
A Model for Cretaceous and Tertiary Extension of the Ross Sea, Antarctica.
The timing and magnitude of Ross Sea extension has remained largely problematic. Outstanding questions include, did Tertiary extension of the Victoria Land Basin (VLB) affect the Central and Eastern Basins, why is there so much Oligocene RSS-2 in the Ross Sea basins, and what accounts for the necessary accommodation space? We computed a 1-D backstripped subsidence model for the Central Trough and Eastern Basin to infer periods of extension. Unit thicknesses were obtained from interpreted MCS depth sections in the deepest part of the basins for the analysis. Stratigraphic units were decompacted and tectonic subsidence calculated. Estimated corrections for paleobathymetry and eustatic sea level changes were applied. Our results support two extensional episodes throughout the Ross Sea, a Cretaceous episode and an early Tertiary episode. Cretaceous extension occurred at a stretching factor of ~2 and Tertiary extension occurred at ~1.5 to 2 in the Central Trough and portions of the Eastern Basin at this time. Tertiary stretching factors in the VLB were 2 to 3. All extension was completed by the first deposition of RSS-2 above unconformity RSU6 (~30 Ma). If Oligocene strata were deposited during extension, syn-rift characteristics are expected. However, these strata are not faulted or deformed in the central and eastern Ross Sea. Tertiary extension was likely simultaneous with seafloor spreading along the Eocene Adare Trough (43-26 Ma) northwest of the Ross Sea. Approximately 180 km of east-west extension due to this spreading could have extended Ross Sea basins. The VLB is 130 to 150 km wide, requiring that remaining Adare Trough extension is accommodated elsewhere, possibly in the Central Trough and the Eastern Basin. An alternative model with only Cretaceous extension east of the VLB, limiting Tertiary extension only to the VLB [Karner et al, 2005 EPSL], requires that most thermal subsidence throughout the Ross Sea predates Oligocene sedimentation. This timing requires that Oligocene unconformity RSU6 formed many of hundreds of meters below sea level, implying that RSU6 is not a wave-cut feature and that other processes, such as glaciation or sea level changes, formed the unconformity. We intend to expand our 1-D model into a 2-D model and apply decompaction corrections and subsidence calculations to a grid of the Ross Sea in order to determine how all the basins respond in 2-D.